Click on the "Public" option from the home page and search for a model to design around.
At the top, select 'Make a Copy'
Along the top bar, select 'Offset'. Select each face that you want to be a part of the phone case. Offset by a small amount (~1mm)
Along the top bar, select 'Thicken'. On the side bar, select the offset you just made. Make the phone case as thick as you want!
Start a new 'Sketch' and choose the back of the phone as the sketch surface. Draw a new sketch, or paste in a sketch you already have!
Select 'Extrude' and choose the sketch you just made. Don't extrude it too much, or it will pop out too much!